Delta3,
I could possibly believe that severe turbulance might cause a condition of +0.5g but that is possibly the limit, due mostly to the high blade loading of all helicopters, but I am quite sure that the mast bumping accident requires much lower than 0.5g to initiate the accident sequence.
Therefore, I am quite convinced that NO Mast bumping can be caused to an innocent pilot by the guilty aircraft. The FAA thinks so, as well, otherwise they would not certify teetering rotors.
gentleban,
Remember that the torque of the main rotor is countered by the force of the tail rotor thrusting sideward, as well as the force of the vertical fin, also thrusting sideward.
The tail rotor thrust and the tail fin thrust are both fairly high, quite above the roll centroid of the helo, so they produce a rolling motion to the right. This rolling moment is countered by a slight cyclic trim to the left. But at zero g, there is no cyclic control. With no cyclic control, nothing opposes this tail rotor roll. If the pilot uses cyclic to try and stop it, he can actually cause the mast bumping by putting in excessive cyclic.
I would avoid any thoughts of pulling or stomping anything if you get near zero g. It is my belief that most mast bumpings are probably the result of panicy pilots abusing the controls at zero g, and the excessive control positions they achieve that CAUSE mast bumping.